Generally, an electrophotographic image forming process or an electrostatic printing process includes the steps of (1) forming an electro-conductive image (i.e., a latent image) or an electrostatically charged image corresponding to a recorded image on the surface of an electrostatic recording medium, for example, an OPC (Organic Photo-conductor) drum, (2) developing the latent image with a charged toner, (3) transferring the developed toner image onto a recording material, such as a paper or a recordable film, and (4) fixing the transferred image on the recording material with a heat-compressing roller. The image-forming process such as the electrophotographic image forming process or the electrostatic printing process has advantages in that a printed or copied matter can be obtained with a high speed, the image formed on a recording material is stable, and an image-forming device is easy to manipulate. Therefore, the image-forming process has been widely used in the fields of copiers and printers.
The toner for the developing process can be classified into a one-component toner, a two-component toner, etc. The two-component toner includes a binder resin, a coloring agent, an electrification control agent, various additives, and magnetic substances for developing the latent image formed on a drum and transferring the developed image. In general, the toner is produced in the form of particles by melting, kneading, and dispersing the toner components, and then finely pulverizing and classifying the kneaded components. The binder resin, which is one of the main components of the toner composition, should have desirable properties in dispersing the coloring agent, and in a fixing property, an offset-preventing property, a storage-stability, and an electrical property. The binder resin also should have a good transparency, and produce a clear image even when a small amount of the coloring agent is used. Further preferably, the binder resin should have a wide color expression range, improve the image density of a copied or printed image, and be environmentally desirable.
As the binder resin, polystyrene resin, styrene-acrylic resin, epoxy resin, and polyamide resin have been conventionally used. Recently, a polyester resin is more commonly used as the binder resin due to its superior fixing property, good transparency, and so on. The polyester resin for the binder resin generally uses bisphenol A or its derivatives as the alcohol component of the polyester resin. However, bisphenol A is not desirable in an environmental aspect. Therefore, various researches have been carried out to produce a polyester resin having desirable offset-preventing property, low-temperature fixing property, sharp-melting property, blocking-preventing property, electrical charging property, pulverizing property, storage stability, transparency, image-forming property, and so on without using bisphenol A or its derivatives. For example, in Korean Patent Laid Open Nos. 10-2005-51543 and 10-2005-6232, resins without including bisphenol A or its derivatives are disclosed. However, the resins include relatively large amount of ester groups, which are polar groups, so the resins absorb much water at high temperature and high moisture condition. Therefore, in the cartridge of high temperature and high humidity condition, the initial print image would be good, but the print image would become deteriorated as the printing proceeds since the toner absorbs much water and the electrical charging property of the toner becomes deteriorated. In the above-mentioned patents, in order to improve the offset-preventing property of a toner, the resin was cross-linked or gelated (i.e., increase of insoluble content of resin in tetrahydrofuran (THF)) by using polyvalent acid component and/or polyhydric alcohol component. However, the cross-linked or gelated part of the polyester resin is easily degraded due to a high shear stress when extruded and kneaded for the production of a toner, which deteriorates the offset-preventing property of the toner. The drawbacks of the polyester resin without bisphenol A or its derivatives have not been solved yet. In Japanese Patent Laid Open (kokai) No. H11-305485, cyclohexane dibasic acid or diol is introduced in the production of a polyester resin in order to improve the fixing property and the color tone reproduction of a toner. In the patent, cyclohexane dicarboxylic acid is used in the amount of more than 20 mol %, and thus, the produced polyester resin has the glass transition temperature (Tg) of less than about 58° C., which deteriorates the storage stability and the pulverizing property of a toner. Also, in the patent, the deterioration of the fixing property of a toner due to the amount of the cyclohexane dibasic acid or diol in the resin was not considered.
In preparing a polyester resin for toner, a catalyst such as germanium based catalyst, antimony based catalyst, and tin based catalyst and so on has been used. However, because the catalyst should be used in large amount due to a low catalytic activity of the catalyst, the catalyst is not desirable in an environmental aspect. The conventional catalyst deteriorates the transparency of a polyester resin due to its characteristic coloring property (for example, the antimony based catalyst has a gray coloring property). Therefore, titanium based catalyst is tried to improve the catalytic activity and the transparency of a polyester resin, and the titanium based catalyst includes tetraethyl titanate, acetyltripropyl titanate, tetrapropyl titanate, tetrabutyl titanate, polybutyl titanate, ethylacetoaceticester titanate, isostearyl titanate, titanium dioxide, TiO2/SiO2 co-precipitates, TiO2/ZrO2 co-precipitates, and so on.